Quality control at site; slump test

Quality control of concrete at site (IS 456 Clauses 15 and 16)

1. Materials: test the cement (IS 4031), aggregates (IS 2386: grading, bulking, moisture), water and admixtures before use; store them properly (cement off the ground, aggregates kept separate).
2. Mix proportioning: use a design mix with weigh batching; correct for moisture, bulking and absorption; keep the w/c fixed.
3. Mixing and transport: use mechanical mixers (mixing for at least 2 min); avoid segregation; place within the time allowed.
4. Workability checks: slump test on every batch or truck.
5. Placing, compaction and curing: proper compaction (vibrators), clean formwork, cover blocks, and curing for at least 7 days (10 days for PPC; IS 456 cl. 13.5.1).
6. Sampling and strength tests: 150 mm cubes, sampled as per IS 456 cl. 15.2.2 (1 sample for 1-5 m$^3$, 2 for 6-15 m$^3$, 3 for 16-30 m$^3$, and so on); each sample gives a test result equal to the average of 3 cubes, tested at 7 and 28 days. Acceptance by cl. 16.1 and Table 11 (for M20 and above): the mean of any 4 consecutive test results $\geq f_{ck} + 0.825 s$ (or $f_{ck} + 4$ N/mm$^2$, whichever is greater), and every individual test result $\geq f_{ck} - 4$ N/mm$^2$ ($f_{ck} - 3$ N/mm$^2$ for M15).
7. Records, supervision, and calibration of equipment; core tests or NDT (rebound hammer, UPV) in case of doubt.

Slump test in detail (IS 1199)

Apparatus: slump cone (bottom diameter 200 mm, top 100 mm, height 300 mm), 16 mm tamping rod 600 mm long, base plate, scale.

Procedure: place the cleaned, moistened cone on the base plate; fill it in four layers, each about one-quarter of the height; tamp each layer with 25 strokes of the rod; strike off the top; lift the cone vertically within 5-10 s; measure the subsidence (the slump) from the top of the mould to the highest point of the subsided concrete.

Results: true slump (valid), shear slump (repeat the test) and collapse slump (too wet). Typical slump values: 25-75 mm for lightly reinforced sections, 50-100 mm for heavily reinforced sections. The test is quick, but not suitable for very dry or very wet mixes. A variation of more than ±25 mm from the design value indicates a change in water content or grading.